1. Field of the Invention
The present invention relates to a strobo device wherein a flash discharge tube is connected in series to an insulated gate bipolar transistor which controls the luminous operation of the discharge tube, and more particularly to a strobo device having effective characteristics in a voltage supply system to the flash discharge tube in case of high-speed repeating luminous emissions.
2. Description of the Prior Art
A strobo device utilizing an Insulated Gate Bipolar Transistor (hereinafter called the IGBT) is known as disclosed in U.S. Pat. No. 4,839,686.
As shown in FIG. 12, the device consists of a DC high voltage power supply 1 which is known DC-DC converter circuit, a main capacitor 2 which is charged by the power supply 1, a constant-voltage circuit 3 which is placed adjacent to the power supply 1 and supplies a flash control circuit 7 described later with a constant-voltage, a known trigger circuit 4 which triggers a flash discharge tube 5, a control circuit 6 which is connected to a control means 8 in a camera body, and receives and sends different output signals such as a trigger signal to actuate the trigger circuit 4, a flash control circuit 7 which controls an on/off operation of the IGBT connected in series to the flash discharge tube 5 and also controls a light emission of the flash discharge tube 5, and a multiplying circuit 9 which applies a doubled voltage of the charged voltage of the main capacitor 2 between main electrodes of the flash discharge tube 5.
In the above device, when the DC high voltage power supply 1 is started by turning a switch Sw on, the main capacitor 2 and the multiplying capacitor 9a are charged forward by a high voltage output by the DC high voltage power supply 1. A capacitor for a power supply Ce supplying power to the control circuit 6 is charged by a low voltage power supply E, and a capacitor 3a of the constant-voltage circuit 3 is also charged. In this way, the control circuit 6 is actuated and the flash control circuit 7 is ready for the luminous operation.
At this time, the control circuit 6 outputs a high level signal by inputting a flash start signal from the control means 8 and turns on transistors Qa and Qb in the flash control circuit 7. Then the IGBT is turned on by charged voltage of the capacitor 3a. Thus the charged voltage of the multiplying capacitor 9a superimposed on that of the main capacitor 2 is applied between the main electrodes of the flash discharge tube 5 and actuates the trigger circuit 4. In this way, the flash discharge tube 5 flashes by using the charged electricity of the main capacitor 2.
In the above flashing procedure, when a flash stop signal is input in the control circuit 6 by the control means 8, the control circuit 6 is actuated, outputs a high level signal from an output terminal Ob and turns on transistors Qc and Qd of the flash control circuit 7, which turns off the transistor Qb and the IGBT which have been on, resulting in stopping the luminous operation of the flash discharge tube 5.
Described above is a basic function of the conventional device shown in FIG. 12. During the luminous operation by the device, a doubled voltage of the main capacitor 2, a superimposed voltage of the charged voltage of the multiplying capacitor 9a and the main capacitor 2, is applied between the main electrodes of the flash discharge tube 5. Thus the strobo device of the invention differs from the conventional device, in which the luminous operation is stopped by a commutating capacitor, in preventing the flashover, thereby obtaining a small-size device realizing repeating luminous operations at a high speed.
However, in the process of the high-speed luminous emission, when the period is over a specified one, for example, more than several ten Hz in the device shown in FIG. 12, the next luminous emission may be actuated before the multiplying capacitor 9a is sufficiently charged. In this case the multiplying circuit 9 may not function appropriately, resulting in failing to flash the flash discharge tube 5, and flash failures are likely to happen.
For example, it is evident from the circuit structure that the multiplying capacitor 9a is charged only when a cathode of the flash discharge tube 5 has a low potential.
FIGS. 13a, 13b and 13c are diagrams showing voltage waveforms and flash waveforms at predetermined points, points A and B in FIG. 12, in the luminous operation in a conventional device. As shown in FIG. 13a, when a high level voltage is applied to the points A, a gate of the IGBT, at a time T1 and the applying voltage is stopped at a time T2, the IGBT is turned on and then off as mentioned above, therefore, the flash discharge tube 5 flashes as shown in FIG. 13c. A potential of the point B, which is a cathode of the flash discharge tube 5, (a cathode potential) in the above procedure once falls sharply at the time T1, then rises sharply at the time T2 and gradually falls hereafter as shown in FIG. 13b.
It is known that the flash discharge tube 5 doesn't return to a steady state immediately after the IGBT is turned off at T2 but is in the ionization state in which the tube doesn't flash. Once the flash discharge tube 5 flashes, the cathode potential is kept at a high level until the flash discharge tube 5 returns to the initial state through the ionization state, even if the power supply is stopped. Accordingly, the multiplying capacitor doesn't start to be charged from the time T2. While the flash discharge tube 5 is in the ionization state and the cathode potential is kept at a high level, the multiplying capacitor 9a does not become charged. Therefore, even if a high level voltage is applied to the point A at the time T3 before the cathode voltage does not return to level 0, the charged voltage of the multiplying capacitor 9a can not be applied to the flash discharge tube 5, as shown in FIG. 13a by a broken line.
Moreover, the multiplying capacitor 9a has an appropriate electric charge time constant, therefore, it is not sufficiently charged during the time constant even after the cathode potential returns to level 0. Therefore, the multiplying circuit 9 can not fully function before the time constant is completed, even if the next luminous operation is carried out. For example, when the flash discharge tube 5 flashes with a period range of more than several ten Hz so many times that the charged voltage of the main capacitor 2 is reduced, the luminous failures occur in the flash discharge tube 5 and the luminous emission disadvantageously can not follow the desired period in the above period range.
It is known that in the case of a very high period, more than the above period range, the flash discharge tube 5 flashes very easily, resulting in no luminous failures, as the next luminous operation is carried out when the flash discharge tube 5 can flash without being triggered.
On the other hand, in order to miniaturize the flash discharge tube and increase the quantity of flashing light, a method to make the impedance high by a high inside gas pressure is known. It is also known that a starting voltage of the flash discharge tube can be risen by this method. In addition, when the luminous emissions are repeated at a high speed, a characteristic of the outgoing radiation is deteriorated by miniaturization, and a characteristic of storing heat is risen by the high impedance, which further rises the starting voltage. From these points of view it is a big disadvantage for the flash discharge tube that the multiplying circuit can not be expected to function.